1. Field of the Invention
The present invention relates to an optical scanning device arranged in an image forming apparatus.
2. Description of the Related Art
Conventionally, image forming apparatuses such as an optical printer, a digital copying machine, an optical plotter, and the like include an optical scanning device that uses a light beam to scan a target surface for scanning. Regarding such image forming apparatuses, the demand for low manufacturing cost and robustness against temperature fluctuation is on the increase in recent years.
With the advances in high-precision processing technology, it is now possible to manufacture a robust optical scanning device having fewer components at low cost by using microstructure optical elements such as a diffraction lens, a phase shifter, a sub-wavelength structure (SWS), and the like.
By using a diffraction lens in an optical scanning device, it is possible to enhance the high-precision optical properties as well as downsize the optical scanning device.
Japanese Patent Application Laid-open No. 2005-258392 discloses an optical scanning device that includes a semiconductor light source, a coupling optical system, a converging optical system, an optical deflector, and a scanning optical unit. A light beam emitted by the semiconductor light source undergoes coupling while passing through the coupling optical system. Subsequently, upon passing through the converging optical system, the coupled light beam is collimated into a substantially parallel light beam along a main scanning direction and converged near a deflecting surface of the optical deflector along a sub-scanning direction. The optical deflector deflects the converged light in the main scanning direction and the deflected light is re-converged in the scanning optical system. Meanwhile, each lens arranged in the coupling optical system is a resin lens having at least one diffractive surface.
Japanese Patent Application Laid-open No. 2002-287062 discloses a laser scanning device that includes a laser light source, a light source optical system, an optical deflector, and a scanning optical system. The laser light source emits a laser light through the light source optical system. Upon passing through the light source optical system, the emitted laser light is collimated into a substantially parallel light beam along a main scanning direction and converged near a deflecting surface of the optical deflector along a sub-scanning direction. The optical deflector deflects the converged light in the main scanning direction and the deflected light is re-converged in the scanning optical system. Meanwhile, the light source optical system includes an optical element made of resin. The optical element has a light reflecting surface and a light output surface. The light reflecting surface has at least one face without a rotation symmetric axis. The light output surface is a two-faced surface where each face is a diffractive face. When a wavelength shift occurs in the light source optical system, the diffraction angle of each diffractive face of the light output surface varies in a mutually opposite direction.
Japanese Patent Application Laid-open No. 2004-126192 discloses an optical scanning device that includes a light source, a pre-deflection optical unit, an optical deflector, and an imaging optical system. The pre-deflection optical unit guides a light beam emitted from the light source toward the optical deflector. The image optical system guides the deflected light beam toward a target surface for scanning. The target surface for scanning is scanned based on the rotational movement of the optical deflector. One or more surfaces of the pre-deflection optical unit have a diffractive property. Moreover, the pre-deflection optical unit is configured to satisfy a particular condition by using a particular expression that includes a focal length, a beam spot diameter, an oscillation wavelength, an optical power, and a dispersion value of the pre-deflection optical unit.
Meanwhile, a diffraction lens can be fabricated to have a minor step for causing a phase difference of 2π. Such a diffraction lens can also be configured to have refractive and converging properties identical to a refractive lens. However, the property that distinguishes a diffraction lens from a refractive lens is strong negative dispersion, which can be used to achieve temperature compensation. More particularly, temperature compensation can be achieved by obtaining a suitable combination of the negative dispersion of a diffraction lens and wavelength shift of a light source that occurs due to temperature fluctuation of the corresponding optical system.
That is, temperature compensation is achieved when the variation in the optical properties due to temperature fluctuation of the optical system and the wavelength shift of a light source occur in a fine balance. Thus, when a laser light source such as a semiconductor laser diode is used for emitting light, it is necessary to take into consideration the deterioration in geometric aberration due to the wavelength shift thereof that occurs because of various reasons such as the difference in wavelength of each light source element in the laser light source, the modehop during the emission of light from the laser light source, the difference in wavelength of each light emitting part in an array element of the laser light source, and the like. This is an inevitable issue that needs to be addressed when the wave properties of the light are subjected to geometric aberration correction.
Only when necessary geometric aberration correction is performed, the phase shifter is able to perform wave-front control. In other words, if geometric aberration occurs due to temperature fluctuation or positional errors while installing components in an optical scanning device, then the phase shifter cannot perform wave-front control. That can lead to deterioration in the optical performance of the optical scanning device.
To avoid such a problem, it is necessary to incorporate the function of geometric aberration correction in an optical scanning device that includes a phase shifter. For that, a variety of integrated diffractive optical elements are proposed that have a composite surface for performing geometric aberration correction as well as other functions. However, following problems occur with respect to such integrated diffractive optical elements. Firstly, it is difficult to process and mold an integrated configuration of a multistep structure and a two-step structure and the level difference in each orbicular zone is different. Secondly, in the case of manufacturing a low cost optical scanning device, the emphasis is given not on the functionality or precision of an independent optical element but on the degree of freedom for adjusting positions of the optical elements at the time of mounting. In such a case, it is difficult to independently adjust an integrated diffractive optical element that also has refractive properties and can perform wave-front control. Otherwise, there is a possibility of hampering the degree of freedom for adjusting the other optical elements. A phase shifter that minutely performs the wave-front control can be used on the premise that there is no fluctuation in the point of focus. However, when a diffraction lens is used to achieve a stable point of focus, it becomes necessary to resolve the issues regarding wavelength fluctuation.